Inflation: Artificial expansion of the money supply.
Because of the way inflation is used to redistribute wealth towards those who have politically-favored connections (to the counterfeiting operation) this hidden tax is extremely regressive. It penalizes the poor (those who have none of the political connections enjoyed by the well-connected) to a much greater extent than the rich because the rich are more connected—directly or indirectly—to the early stages of the injection of the new funny money into the system.
Just like all taxes take some portion of the individual's wealth at the time of the transaction the result is a diminished total value of income that can be used for living. But everyone knows how taxes are constantly taking their wealth. The common saying is "Only two things are certain: death and taxes!" Inflation is a hidden and extremely regressive tax that most people do not know enough about to be able to muster a concerted resistance.
And that is just the way the State likes it. Keep the people ignorant about this hidden tax because it then can continue to be used to extract wealth from the people and because a foolish population can easily be told that inflation is caused by all sorts of bogeymen; deflecting the ire of the masses (a regressive tax hurts the masses the most) away from the counterfeiting operation of the State.
Inflation is a hidden and extremely regressive tax which means that it is unjust and it is theft that is especially targeted at the masses.